1. Field of the Invention
The invention relates to a three-phase transformer, and more particularly to an improved three-phase transformer with windings which provide discrete voltage levels form a plurality of taps. Such transformer can be used in many applications such as a reduced voltage motor starter.
2. Description of the Prior Art
There are many applications which require a means to adjust the amplitude of a balanced three-phase AC voltage source. A balanced three-phase source is one which has available three equal voltages which are 120.degree. out of phase with each other. Two examples of applications are a regulator for utility voltage, and a reduced-voltage starter for AC induction motors. FIG. 1 shows an example of a previous approach for a reduced-voltage starter in which a star-connected three-phase transformer winding is provided with a plurality of taps on each phase. For large motors it is desirable to employ reduced-voltage starting due to starting current restrictions. Although FIG. 1 shows the case of four taps per phase, any number is possible. A common method of applying reduced-voltage starting to an AC induction motor is through such an autotransformer. After the input terminals R, S, and T of the transformer have been energized, the taps are connected by closing the appropriate switches to gradually increase throughput voltage, for example 50%, to the final AC induction motor operating voltage, which may be 100% of the transformer output. The degree of control in the motor operation is directly related to the number of taps; the more taps and switches, the more precise the transformer output voltage control. FIG. 1 shows the transformer winding configured as an autotransformer, but it could also be configured as a conventional isolation transformer. The autotransformer shown consists of a single coil per phase linked by a magnetic circuit. Taps are provided such that, with input voltage being applied to one set of taps, output voltage may be taken from any other set of taps. The fixed input voltage terminals are labeled R, S, and T, while the adjustable output terminals are labeled U, V, and W.
For each phase in FIG. 1, a set of switches is provided to allow the corresponding output terminal to be connected to any one of the corresponding taps. While conventional switches are shown in FIG. 1, any type of switch can be utilized provided it is capable of blocking voltage of either polarity when open, and capable of conducting current of either polarity when closed. FIG. 2 shows alternative switch types typically implemented with semiconductors.
For the case of four taps per phase shown in FIG. 1, there are a total of twelve switches (SWU1-SWU4, SWV1-SWV4 and SWW1-SWW4). To avoid short-circuiting the winding, only one switch can be closed in any phase at any time. If the output voltage is to remain balanced, the same switch must be closed in each phase. For example, if switch SWU2 is closed then switches SWV2 and SWW2 must also be closed. Thus even though twelve switches are present, the circuit of FIG. 1 can provide only four distinct levels of balanced output voltage to terminals U-V-W.
FIG. 3 shows another previous approach, which improves on FIG. 1 by providing the same number of output levels with fewer switches. In FIG. 3 the transformer winding is connected in a mesh instead of a star configuration. Also the winding for one phase is omitted. This configuration is known in the art as an open-delta connection. The remaining two windings must support line-to-line voltage, which is 73% higher than the line-to-neutral voltage supported by the three windings of FIG. 1. It is also necessary to reverse the coupling polarity of one of the remaining windings as compared to FIG. 1, as shown by the polarity dots, in order to maintain equal flux in all paths of the magnetic circuit. The output for the phase with the missing winding is connected directly to the corresponding input for the autotransformer case. In FIG. 3 the winding for output terminal V is missing, and output terminal V is connected directly to input terminal S. The approach of FIG. 3 requires only eight switches (eliminating switches SWV1-SWV4) to provide four distinct levels of output voltage. However, this approach is still constrained by the need for matching switches to be closed in each remaining phase.
The large number of switches or connections needed to obtain a small number of levels can discourage the use of either FIG. 1 or 3. For some transformer applications, such as reduced-voltage starters or regulation of utility voltage, it is often desirable to have more levels available to give more precise control.
What is needed then is a methodology whereby the output voltage levels of a transformer or the like can be increased to allow more precise control without inordinately adding to its complexity and cost.
It is therefore an object of the present invention to improve the adjustability of transformer output voltages while minimizing the number of switches or connections required.
It is a further object of the present invention to decrease the number of transformer taps needed to achieve a desired degree of adjustability.
It is another object of the present invention to avoid the constraint that matching taps must be connected in each phase.